Tie your new habit to something you already do.
For example, I've always wanted to be able to a 'normal' pushup. I decided I would start with 2 "on your knees" pushups everyday before taking my shower. To remember to do them, I made my cue turning on the water.
So each day, after I turned on the water for my shower (already a habit) I walked back into the bedroom and did my 2 pushups. I started small so I wouldn't have an excuse not to do it -2 pushups only takes a few seconds.
Since I started this small habit, and tied it to a cue, it became part of my daily routine. Every so often I'd add a pushup increasing the number I'd do.
Now I do a mix of 'normal' pushups, modified pushups and chaturanga pushups!